Epoxy resin compositions that contain epoxy resins exhibit excellent workability, and cured products obtained therefrom exhibit excellent electrical characteristics, heat resistance, adhesive strength, moisture resistance (water resistance), formability and the like. As a result, epoxy resin compositions have been widely used in the past as adhesives used to bond a variety of substrates in fields such as electrical/electronic components, vehicle components, electrical equipment, fiber reinforced plastics (FRP), sporting goods, structural materials and coating materials.
As materials related to this type of adhesive, epoxy resin compositions comprising, for example, an epoxy resin, a urethane-modified epoxy resin partially containing blocked isocyanate groups, a liquid rubber and a curing agent have been proposed (For example, see Japanese Unexamined Patent Application Publication No. H05-148337A). By heating the epoxy resin composition at a high temperature, some of the blocked isocyanate groups in the urethane-modified epoxy resin dissociate and react with hydroxy groups to form a compound containing hydroxy groups, and this compound reacts with, and bonds to, hydroxy groups in the epoxy resin, thereby exhibiting improved shear adhesive strength and peel strength as a structural adhesive for a vehicle or the like.
However, the epoxy resin composition disclosed in Japanese Unexamined Patent Application Publication No. H05-148337A is brittle, and therefore has problems such as poor adhesive properties and shock resistance. Therefore, a cured product obtained from the epoxy resin composition disclosed in Japanese Unexamined Patent Application Publication No. H05-148337A breaks easily when subjected to shocks.
In order to make up for these drawbacks in the above-mentioned epoxy resin composition, one method is to add an elastomer such as a urethane resin or a rubber as a flexible component, but it is still the case that there are few products having satisfactory shock resistance and an epoxy resin composition that exhibits excellent shock resistance while maintaining adhesive strength has yet to be found as a structural adhesive.